Uptake of organic anions by the liver has carrier-mediated kinetics. 35S-BSP binds specifically and with high affinity to rat liver cell plasma membrane preparations (LPM), and this binding is eliminated by trypsin preincubation of LPM. Using a photoaffinity probe, a 55,000 dalton LPM binding protein was identified and purified by affinity chromatography on BSP-agarose gel. Antibody to this protein has been prepared in rabbits, and we plan to set up a radioimmunoassay to quantitate this protein in liver subcellular preparations as well as in membrane preparations from other organs. Organic anion transport in isolated perfused rat liver following infusion of antibody will also be studied. Several rat models in which the influx rate of organic anions is reduced have been characterized by analysis of multiple indicator dilution studies performed in isolated perfused liver. These models include regeneration, fasting, and nafenopin treatment. Studies of the interaction of 35S-BSP to LPM from these models may reveal changes in ligand-protein interaction.